1. Field of the Invention
This invention relates to a natural rubber composition having improved properties in the unvulcanized state as well as in the vulcanized state. More particularly, the invention relates to a natural rubber composition comprising a natural rubber and a specified liquid polyisoprene rubber.
2. Description of the Prior Art
It is one of the recent trends in the rubber industry to use low molecular weight plasticizers such as process oil, lanolin and dioctyl phthalate so as to decrease the degree of plasticity of unvulcanized rubber compounds, to increase processability thereof for the purpose of labor- and energy-savings. The use of the above-mentioned low molecular weight plasticizers causes a decrease in the viscosity of unvulcanized rubber compounds and an increase in the flowability thereof; thus contributing greatly to labor- and energy-savings in the process of manufacture of rubber goods. The use of these plasticizers also decreases the mechanical properties, typically the green strength, for example, of rubber compounds in the unvulcanized state. This decrease in green strength causes difficulties in the molding step and at the same time a decrease in the properties of the vulcanizates, such as hardness and modulus, are inevitable, and therefore, the vulcanizates are not always satisfactory in for the manufacture of such rubber goods as belts, hoses, tires, rolls, vibration absorbers and rubber shoes, where severe and strict requirements are put forth with respect to performance of the products. Moreover, in cases where metal reinforcements are often used, such as in the case of belts, hoses, tires and vibration absorbers, the use of lower molecular weight plasticizers such as mentioned above is undesirable for reasons which concern adhesion to the metal. In general, vulcanizates containing these plasticizers are inferior to those which do not contain such plasticizers.
In order to solve this rather contradicatory problem, namely simultaneously increasing flowability in the unvulcanized state and processability, green strength, a rubber composition has been proposed wherein a low molecular weight polyisoprene rubber with an intrinsic viscosity in the range of 0.1 to 1 incorporated as low molecular weight plasticizer into a solid rubber, cf. for example British Pat. No. 990,439; U.S. Pat. No. 3,335,202. Further, British Pat. No. 1,111,978 discloses polyisoprene rubbers having various intrinsic viscosity distributions between 0 and 20 dl/g. In these methods, however, the adhesion of the rubber to the metal reinforcements is not adequate.
Further, it is known to introduce a polar group, for example maleic anhydride, into a solid cis-1,4polyisoprene rubber in order to improve the green strength, cf. for example U.S. Pat. No. 3,567,691; U.S. Pat. No. 3,507,736; U.S. Pat. No. 3,664,248; U.S. Pat. No. 3,887,527; U.S. Pat. No. 3,909,473; U.S. Pat. No. 3,898,193; British Pat. No. 1,404,291; British Pat. No. 1,406,935 and British Pat. No. 1,211,795. These methods, also, are not fully satisfactory in respect to flowability and processability.
Natural rubber which has been used with large quantities possess excellent processability and mechanical properties. Especially, with respect to processability, no synthetic rubber yet produced is superior to the natural rubber. Certainly, cis-1,4-polyisoprene rubber (IR), which has a structure similar to that of natural rubber, is superior to other synthetic rubbers in processability, but even this rubber does not equal natural rubber in green strength which influences the processability. Green strength is the basic required property for handling an unvulcanized rubber compound. If the green strength is poor, the unvulcanized rubber compound will undergo the state of cold-flowing before vulcanization. Especially, in a case of manufacturing hard rubber articles such as tires or belts, still higher green strength is needed for the process of doubling for carcass or forming unvulcanized body for tires or belts.
As mentioned above, natural rubber has a high green strength. However, it is not satisfactory in all respects. Namely, in some cases as mentioned above, a higher green strength is desirable, and in the case of using plasticizers or softeners to improve the flowability of the rubber compound on calendering or extruding, the green strength decreases while the plasticity increases.
Thus, a need continues to exist for a natural rubber composition which is easy to work in the unvulcanized state and yet possess excellent properties when vulcanized physically.